1. Field of Invention
The present invention relates to RF (radio frequency) networks and more specifically to RF networks having variably controlled inductors for selective tuning.
2. Background Description
In many RF network applications, transmission source signals are used to electrically excite an antenna for propagating electromagnetic radiation. As with RF reception circuitry receiving the radiated transmitted signals, the transmission RF network is typically tuned to a particular transmission frequency within a frequency bandwidth. However, the RF transmitter may have an RF signal source operated at many different frequencies during use. Thus, some RF networks may require variable components. For example, an antenna matching RF network may have variable inductors which are electronically controlled by peripheral electronic circuitry. In such instance, the transmission antenna is coupled to the RF signal source through a tuned RF network having a selectable resonant impedance matching frequency. As the operative source frequency is changed, the tuned circuit resonant frequency should change correspondingly so as to obtain maximum energy transfer and optimum impedance match between the signal source and the transmission antenna. Conventionally, the tuned RF matching network will maintain a fifty ohm impedance match between the signal source and the antenna over the selected frequency range for efficient energy transfer to the antenna.
In order to permit rapid electronic selection and changing of the transmission or reception frequency, the values of tuned components such as inductors or capacitors must be changeable with an appropriate fast speed. These value changes are typically controlled by external electronic control lines. Thus, the tuned components should have variable values which are switched at high speed and selectable by external electrical control signals.
A typical implementation of a variably tuned network includes a plurality of discrete fixed inductance value inductors connected in series. Each of the inductors has a respective parallel short circuit switch which is opened or closed so as to respectively switch "in" or "out" the fixed discrete inductor within the series connection. The values of those inductors which are switched in, that is, when their respective short circuit switch is open circuit, are summed to a total inductance value. The maximum inductance value results when all the inductors are switched in, that is, with an open circuit switch position for each of their respective switches. The minimum inductance value which is optimally negligible results when none of the inductors are switched in, that is, when their respective switches are in a closed circuit switch position thereby shorting the inductors.
The switches must have a suitable switching speed to enable rapid selection of different operating frequencies especially for computer controlled frequency "hopping" applications. Certain types of mechanical switches, e.g. reed switches and vacuum switches, are able to switch at relatively high speeds, however most of these switches have a significant amount of parasitic capacitance associated with their "off" state. Such parasitic capacitance can de-tune a network of series connected inductors as discussed above. Thus, a solid state switch may be used in series with the mechanical switch to block this parasitic capacitance from reacting with the respective inductor.
Further problems with present mechanical switches include their inability to withstand relatively high RF power levels and still have a reasonably fast switching time. A further problem arises in the "settling time" of the switch. In a transmitting operation, the transmitter is typically turned off when switching to avoid arcing and the generation of spurious signals. When summing the time required to turn the transmitter off and then on, that is, the switching and settling times of the mechanical switch, the result is a relatively slow switching means when compared to solid state switches. However, in certain applications, a mechanical switch may be more desirable.
Solid state switches have been used to switch respective inductors in or out of the circuit. The solid state switches used to selectively switch in or out the inductors preferably have a low conducting short circuit "on" resistance and a low open circuit "off" capacitance so as not to affect the resonant frequency of the tuned RF network. Positive Intrinsic Negative (PIN) junction diodes meet these switch requirements. PIN diodes have been used for inductor switching in variably tuned inductor matching networks. It has been found that the PIN diode is well suited for UHF and VHF transmission in circuits of series connected inductors with respective parallel connected PIN diodes because of the long lifetime of the PIN diode. During use with high voltage RF signals conducting through the inductors, the PIN diode exhibits a long on-lifetime based on minority carriers within the device, and remains in conduction thereby shorting out the inductor even during large fluctuation of the voltage of the RF signal.
The high voltage RF signals cause conventional PN junction diodes to turn on and off as the RF signal varies. Hence, these PN diodes are not well suited for switching in high voltage RF signal applications. However, the PIN diodes with an on-lifetime of up to six microseconds, are well suited for high voltage UHF and VHF applications. This long on-lifetime is in contradistinction to the on-lifetime of the PN diode which tends to stop conduction during the negative phase of the high voltage RF signal whereas, the PIN diode with its long on-lifetime, will remain in conduction during the entire signal phase and continue to short out the parallel connected inductor in high frequency applications. For further understanding of various applications using the PIN diode in variable tuning RF networks see U.S. Pat. Nos. 4,564,843, to Cooper; 4,477,817 to Anderson; and 4,486,722 to Landt.
However, the PIN diode is not well suited for low frequency HF applications (three to thirty megahertz), because its on-lifetime is not long enough to keep it turned on during the negative phase of high voltage HF signals. The period of the negative phase of the high voltage HF signal may exceed the minority carrier life-time causing the PIN diode to stop conduction and thereby fail to short circuit the inductor.
Additionally, prior techniques using the PIN diodes, as with other diodes, disadvantageously used a separate DC current source for each separate PIN diode during a forward bias short circuit operation and disadvantageously used a separate reverse bias voltage source for each separate PIN diode during open circuit operation. Certain heretofore applications of the PIN diode have therefore required special reverse voltage drivers and forward current drivers to insure that the PIN diode does not forward conduct in the presence of fluctuating high voltage RF signals when the PIN diode was in the reverse bias off state, or, to insure that the PIN diode does not turn off in the presence of the fluctuating high voltage RF signal when the PIN diode was in the forward bias on state. These separate forward current and reverse bias voltage requirements disadvantageously required many special, large, complicated and costly driver circuits because one was required for each diode. Such biasing source requirements limited the usefulness of PIN diodes in small-sized radios.
A disadvantage of using electrical control lines with the PIN diode is the lack of inherent isolation between the various components in the tuning RF network which conducts the high voltage RF signals and the control circuits such as the forward current and reverse voltage driver circuits of the PIN diodes which may be, for example, controlled by five volt digital computer systems. Such lack of isolation has been found to interfere with the RF signals. These and other disadvantages are solved or reduced by the fast tuning RF network inductor of the present invention.